1. Field of the Invention
The invention relates to the technology of making optical WDM (Wavelength-Division-Multiplexing) multiplexer or demultiplexer using triple-fiber ferrule.
2. The Related Art
The future communication networks demand ever increasing bandwidth. By transmitting several channels in a single optical fiber at different wavelengths, WDM can greatly enhance the retro-reflective transmission capacity of the optical fiber communication networks. A device that combines different wavelength channels into one fiber is a so-called multiplexer, and a device that divides the multiplexed channels into individual ones is a so-called demultiplexer. A variety of technologies have been exploited to develop high performance WDM multiplexer/demultiplexer, including fiber Bragg granting, optical integrated circuit, fused fiber Mach-Zander interferometer, interference dielectric thin film coating technology etc. An international standard wavelength grid has been suggested by the ITU (International Telecommunication Union) for the center wavelengths of the WDM channels. The prior arts use a Y-branch structure of three conventional fiber optic collimators to tune the filter center wavelength to the ITU grid. The center wavelength of such a multiplexer/demultiplexer is made to coincide with a desired ITU wavelength. The disadvantage in the prior arts is mainly in the difficulty of reducing its size to achieve a robust and compact structure thereof. The basic principle and an approach have been disclosed in the copending application Ser. No. 09/255,047 filed on Feb. 22, 1999, in which a robust and compact structure multiplexer/demultiplexer relative to the prior arts is presented by means of V-groove ferrules. Anyhow, it is still desired to have a smaller, less-components structure for the multiplexer/demultiplexer. The related arts can be referred to U.S. Pat. Nos. 4,464,022, 5,204,771, 5,574,596, 5,629,995, 5,712,717, 5,748,350, 5,764,825, 5,786,915, 5,799,121 and 5,808,763.
Therefore, an object of the invention is to provide a multiplexer/demultiplexer with a half-sized structure thereof relative to that disclosed in the aforementioned copending application so as to result in a compact, robust structure thereof, low cost and flexibility in manufacturing, and high stability in hazardous environment.
According to an aspect of the invention, a retro-reflective multi-port fiber optic device includes the triple-fiber ferrule with a GRIN rod lens to couple the light. The triple-fiber ferrule with three fibers thereof is attached to the angled facet of the GRIN rod lens and the WDM filter is attached to the opposite side of the lens. A reflective mirror is attached next to the other side of the WDM filter after alignment. The triple-fiber ferrule is used to couple light in and out of the device, and tune the thin film filter center wavelength to the desired ITU grid. Therefore, the coupled-in light from the common input pigtail fiber of the triple-fiber ferrule will be collimated and transmitted unto the WDM filter wherein the part of the in-pass-band light with regard to the WDM filter will transmit through the WDM filter, and then successively retro-reflect back by the mirror and further transmit through the WDM band-pass filter again. The retro-reflected in-pass-band signal is then coupled into one retro-reflective transmission output fiber/port of the triple-fiber ferrule as a demultiplexed channel. On the other hand, the out-pass-band light with regard to the WDM filter will not transmit through the WDM filter to interact with the mirror, but instead be directly reflected back by the WDM filter and coupled into a reflection output fiber/port of the triple-fiber ferrule to be transmitted to the next stage for further demultiplexing other channels.
Substantially same incidence angles to the WDM filter are required for the transmitted and retro-reflected in-pass-band light beam to achieve the best performance from the WDM filter. In order to maintain the same incidence angle unto the WDM filter, the three pigtail fiber in the triple-fiber ferrule are located on the same circumference with a diameter defined by the common input pigtail fiber and the reflection output pigtail fiber at two opposite ends thereof. Typically, the three fibers are arranged to be positioned at three tips of a T-shape. The center of the corresponding circle is also substantially coincident with the optical axis of the lens. The spacing among the three fibers is selected to match the WDM filter so that the WDM filter center wavelength is tuned to a pre-designated wavelength.
The invention results in especially low cost and high flexibility in achieving a series of different fiber spacing for tuning the center wavelength of the band-pass WDM filter to coincide with that of the ITU grid.